Working Parent or Working Mother?
The question here would be whether the harassment (and whether it is ILLEGAL harassment is another matter) is because you are a single PARENT or a single MOTHER (or father).
Often women are discriminated against/harassed because their (often male) supervisor believes she should be home with the kids while her husband works. This is gender discrimination.
Another option in California is marital-status discrimination. If a supervisor is essentially harassing an employee because of these problems which are a result of that employee being a SINGLE mother, then this is a possibility. In essence, this is discrimination because she is single.
Finally, California has a "kin care" statute that protects employees from discrimination because their child is sick, has to go to the doctor, etc. If this is the cause of the harassing conduct, it is also illegal.
Just some options to get you started...